Twin Triode discussion

I kinda wanted to take Pucho's 12AX7 rant to the next level...

There are tons of different twin triodes out there other then the 12AX7, but the 12AX7 is the standard for various reasons, mainly because they will work with descent results for various applications. But I think we would all agree that what works well for a guitar amp isn't the best tube for a mic pre.

There is lots of talk out there from the guitar guys like, "Use a 12AT7 if you want a cleaner sound", but nothing definitive. I mean there are tons of twin triodes.

12AX7
12AU7
12AT7
12AY7
12AZ7
12BH7
12AH7
6FQ7 / 6CG7
6DJ8
6GU7

And then you have octal twin triodes...

6SL7
6SN7
6SK7

There are so many alternatives to the 12AX7 out there, and you can find most of them NOS for much cheaper. I mean you can buy a NOS 6SL7 for not much more than a Sovtek 12AX7. So other then the socket, what is the difference?

Do specific tubes have diferent places they excell? I am just wandering what's the difference between all these tubes, and where you have used them before and why?

Good Lord you are bassically asking for a tube manual. That's a thick book.
There are a hundread different samples for each of those numbers you listed.
Sounds like fun to me!

You want three things:

High Gain
Low Noise
Good Sound

You can build a little test jig for listening.

yeah, I have the RCA Receiving Tube Manual, but it pretty much just says they are all used for phase inverters on HiFi equipment. I was wanting to know more in a real world explanation, not necessarily on all of them just if you have used one of the odd ball ones before instead of a 12AX7 what was the difference.

ie: this tube had a higher output, and doesn't break up as easy when pushed hard, or this one had a lower output...

Also when someone says high gain I think of a lot of distortion, not a hot output. Even though that isn't really right.

ie: "I had this guy do a high gain mod on my Marshall".

when really all he is doing is changing the cathode follower to another gain stage, thus pushing the phase inverter harder.

a good octal will kick a noval any day. the 6SN7 is a very good tube. it get's even better split up, like 6J5G & 2C22. all cheap tubes.

octal is ol' school, noval is 'modern' post war, sorta. i've concluded that in general those bigger tubes sound better than their smaller cousins. i like the small noval power triodes, like the 12B4A, a lot though. i've never found good use for the ECCXX stuff, not even TFKs. the WE407A & 5687 are nice noval dual triodes though

[quote author="sismofyt"]the WE407A & 5687 are nice noval dual triodes though[/quote]
The 407A is available as the 2C51, 396A and 5670 also with more convenient heater voltages.

Best regards,

Mikkel C. Simonsen

You could build something like this tester only just have an audio amp in there that goes to some phones for evaulating tubes. A lot of the 12A* series have the same pinouts, so you wouldn't have to wire up a bunch of different sockets. This one is $799, so that rules it out for you cheapass DIY'ers. (wait, thats me!)

http://www.vacuumtube.com/toppage31.htm

Actually, if you jsut build a mic pre based on a min 9 pin, you could plug and play different tubes. Try that with transistors.

The only good way to know is to buy some and try them yourself.

You need to buy some to tell if what you read is BS.

With guitar amps look at the tweed fenders with the 12ay7 in the preamp and and also note the different Phase inverters used.

I have built an octal amp based off the deluxe using 6sl7s now the funny thing is that amp can sound good with a 4x12(16 ohm tap) I have. I took it to a lab members house and it did not sound that good with a different speaker(8 ohm tap) too much bass and the mid gain range did not sound that good. I used the russian sls because the nos and pulls I have were microphonic.

The reason for the story is maybe people are mistaking tubes for a different problem. When you build or mod a tube amp you need to be aware of the gain in each stage AND the high and low pass filters beween stages. Do you want mostly preamp distortion or power amp or a good controlled mix that can track pick attack and volume control use(the harder one to do) without to much/little bass with vol changes. You also need to think pickup single, humbucker, bridge, neck, distance of strings to pickup etc..............

My laney vc30 seems to sound better with a mix of russian and china 12ax7s

Now what do you want in a microphone preamp, compressor etc are you going to use negative feedback or run with min neg feedback (triode with a cap bypassed cathode R or fixed bias always some neg feedback from the plate to grid, realy no such thing as no neg feedback in the real world as best I can tell) What you pick will matter. Do you want to "misbias" for a different harmonic spectrum at close to clipping etc etc etc.

is it a CF ?

do you need low Rp?

The cheap 6au6 sounds better(lmuch less noise) than the nos I tried in my 800 microphone type build.

6dj8 types don't sound good in most microphone circuits.

You really need to try the different tubes yourself.

you need to talk specifics.

CJ posted
"Actually, if you jsut build a mic pre based on a min 9 pin, you could plug and play different tubes. Try that with transistors"

you can do that with transistors. Most Si transistors will work fine if the circuit is designed to allow it.

I throw out what little i know here. Somebody with more knowledge can correct/kick my butt if they like.

In the 12A_7 family, the diffs you have are more or less related to gain and plate resistance, Rp. So this can determine which tube you want to use for specific functions. 12AX7 is the default because it has the highest gain. There is a gain cheatsheet here that helped me to understand this better.

I think generally the lower the gain, the lower the Rp, and depending on how the tube is used, this can translate to a lower output impedance for a circuit that doesn't need gain. In many cases, you can sub in a 12AX7 and the ckt will still "work". Its just not "optimised" maybe.

12BH7 (and I think 12AH7) is altogether different from the 12A_7 family. Rp is quite a bit lower than the family on that one. There is a mic pre design with 12AX7 on the input and 12BH7 for the o/p somewhere here and you see the 12BH7 in the o/p stage of some tube comps as well.

Hope that helps a bit.
Charlie

There is a mic pre design with 12AX7 on the input and 12BH7

Click to expand...

Langevin 5116B and UA 1016 are almost the same schematic...

forgot the most important thing; working point, working point and working point. the greateast tonal adjustment can be made by finding the right working point. and it's almost never the one(s) suggested in the datasheet. the 6SN7/6J5 family likes a lot of current, 8-10mA. the 2C22 in particular is a very tough tube and i've run it at +150% for years. also, high impedance tubes tend to sound toobey, while low impedance tubes are more full and dynamic. to me at least

subbing ECC82 for ECC83 etc is very 'wrong'. while it may sound nice, tubes can blow up, especially those wimpy little novals. i've fixed plenty of guitar amps where some guru have put in the 'wrong' tube. but hey they get the right tone, i get food on my table. works

one should ALWAYS check the datasheet to see if the tube can handle the current/voltages and ALWAYS check V/A when the tube is in

also, not all tubes are swappable. many people had nasty surprises with the whole EL34/7027/6L6/6550/KT88/90 thing